Fix invalid javadoc reference on the remote flag in SpanContext

[thswlsqls]

No issue: trivial javadoc markup fix, issue not required (same as #8588).

Description

• The SpanContext class javadoc links {@link boolean remote}, but boolean is a primitive type, so javadoc cannot resolve the reference.
• The generated SpanContext.html renders a collapsed invalid reference block instead of "the remote flag".
• Replace it with {@code remote}, matching how this file already refers to values rather than types. The neighbouring {@link TraceState traceState} resolves fine and is left as is.
• -Xdoclint:all,-missing does not flag this case, so the build stays green while the published javadoc keeps the broken widget.
• This is the only place in the repo that links a primitive type.

Testing done

• Javadoc-only change, test-exempt: no test added.
• ./gradlew :api:all:check passed.
• ./gradlew :api:all:javadoc --rerun-tasks: SpanContext.html no longer contains invalid-tag; the sentence renders as "the remote flag".
• No public API change, so the apidiff file is unchanged.
• Not run: *IT and GraalVM native-image tests (outside check).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.62%. Comparing base (dd811c4) to head (4148ccf).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8694   +/-   ##
=========================================
  Coverage     91.62%   91.62%           
  Complexity    10327    10327           
=========================================
  Files          1003     1003           
  Lines         27138    27138           
  Branches       3187     3187           
=========================================
  Hits          24866    24866           
  Misses         1566     1566           
  Partials        706      706           

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